An existing continuous annealing apparatus that manufactures cold rolled coils includes a heating zone, a soaking zone, and a cooling zone, heats a room temperature strip to a predetermined temperature in the heating zone and the soaking zone, maintains the strip at the same temperature as the predetermined temperature for a predetermined period of time, and cools the strip up to room temperature in the cooling zone. Here, the existing continuous annealing apparatus includes an additional overaging zone or an additional constant temperature maintaining zone after the cooling zone, if necessary, to allow the strip to be subjected to an overaging treatment in the overaging zone or to remain at 600° C. or less in the constant temperature maintaining zone for a predetermined period of time, to be cooled down to room temperature, and to be austempered or martempered.
The heating zone and the soaking zone enable the cold rolled strip to be recrystallized and heated up to a temperature at which the strip may have a predetermined phase. In this case, dissolution of a carbide and recrystallization nucleation and growth of a ferrite structure are performed, and austenite appears at a temperature of A3 or more. These structures all have a polygonal structure. In order to obtain an equilibrium phase suitable for a temperature, the strip may be subjected to crystal grain growth and homogenization treatment by maintaining a certain temperature for a predetermined period of time.
The cooling zone is composed of a slow cooling apparatus and a fast cooling apparatus to allow a single-phase ferrite steel to be rapidly cooled to oversaturate carbon, and to allow a complex-phase steel to be rapidly cooled to obtain a structure containing martensite, bainite, or a remaining austenite in a ferrite matrix structure through fast cooling.
In the overaging zone, oversaturated carbon of a single-phase ferrite steel is precipitated to significantly reduce amounts of solid elements in a short time period, thus improving processability. In the complex-phase steel, the martensite is tempered in the overaging zone, or bainite or bainite including a remaining austenite is formed through heat treatment of bainite at a constant temperature to control strength and ductility. The overaging zone may be removed according to purposes.
The existing continuous annealing apparatus described above has a problem in that steel components cannot be effectively controlled even by adding large amounts of alloy elements by a method of controlling the steel components and making microstructures, due to a polygonal structure obtained by recrystallization from a heating step (Japanese Patent Publication No. 2003-328039, Japanese Patent Publication No. 1984-133329).